This invention relates to methods for treating wood and, in particular, a method for treating whole logs to prevent them from splitting as they are dried to a desired moisture content suitable for a particular use.
In the woodworking industry it would often be desirable to form articles out of large, monolithic pieces of wood. For example, in the furniture industry it would be desirable to turn a single piece of wood on a lathe to produce a table leg. Similarly, it would be desirable to produce decorative and useful items such as bowls from a single piece of wood. Typically, the wood used for such applications is hardwood, for example, alder, aspen, maple, dogwood or cherry, due to the density of such woods and the pleasing appearance of their grain and color characteristics.
Once green wood has been cut it ultimately will dry out unless treated to prevent the loss of moisture, and as it dries it will twist, warp and split. This is principally due to the internal stresses naturally produced as wood dries due to the different amounts of shrinkage that occur in respective dimensions of wood as a result of the wood cell structure. More specifically, much greater shrinkage occurs in a dimension tangential to a log than in a radial dimension, and very little shrinkage occurs in the longitudinal dimension, for a given loss in moisture content. Consequently, when a log is left to dry the substantially greater tangential shrinkage causes the log to split inwardly, thereby eliminating its usefulness for most applications, particularly the fashioning of articles from large, monolithic pieces of wood. Moreover, this problem is exacerbated by nonuniform drying of a log whereby the outer portions dry faster than the interior.
The conventional solution to the foregoing problem has been to cut whole logs into lumber and thereafter dry the lumber. The dried lumber pieces may then be laminated together into large pieces which can be worked on a lathe or otherwise to produce furniture parts and other decorative and useful items. However, not only is the conventional approach expensive but the result is less than satisfactory because of the nonuniform appearance of the resultant product.
Cutting logs into lumber overcomes the splitting problem for several reasons. First of all, the usual stresses resulting from drying wood may be reduced as a result of the shape and size of a board cut from a log, and the position that it assumed within the log. For example, a plank whose widest lateral dimension corresponds to a radial dimension of the log from which it was cut will undergo substantially uniform shrinkage in any one of its lateral and longitudinal dimensions, though the amount of shrinkage in any one such dimension will ordinarily be different from the shrinkage in another of those dimensions, so that as the plank shrinks the respective different amounts of shrinkage in those dimensions will not produce splitting or warpage. In comparison, a plank cut near the perimeter of a log with its widest lateral dimension tangential to the log from which it is cut will undergo less uniform shrinkage in its widest lateral dimension and, therefore, will warp somewhat during drying, but it generally will not split if it is thin enough to bend. Another reason is that the smaller size of lumber in comparison to the log from which it is cut facilitates more nearly uniform drying of the wood. In addition, it has been found that through seasoning of lumber by subjecting it to controlled heat during the drying process, for example in a dry kiln, and in some instances to vacuum, pressure or moisture as well, the drying not only can be accellerated but the quality of the lumber can be improved.
For example, Beach U.S. Re. No. Pat. 4384 discloses a method for seasoning lumber wherein it is subjected to steam at 200.degree. to 280.degree. F. under 30 to 40 psi pressure for five to fifteen minutes and thereafter allowed to dry. Other methods for treating wood, primarily lumber, using various combinations of heat, moisture, pressure and vacuum are disclosed in the following patents: Howard U.S. Pat. No. 900,017; Howard U.S. Pat. No. 1,021,676; Palen U.S. Pat. No. 1,066,523; Fish, Jr. U.S. Pat. No. 1,328,506; Fish, Jr. U.S. Pat. No. 1,328,655; Fish, Jr. U.S. Pat. No. 1,328,657; Fish, Jr. U.S. Pat. No. 1,328,658; Fish, Jr. U.S. Pat. No. 1,328,659; Fish, Jr. U.S. Pat. No. 1,328,661; and Jacobs U.S. Pat. No. 1,333,848. However, none discloses a method satisfactory for preventing green, debarked whole logs from splitting as they are dried, in order to produce a large, monolithic piece of wood suitable for woodworking.